1. Field of the Invention
The present invention relates to a surface acoustic wave device for use in a resonator, a band-pass filter, or other suitable component. More specifically, the present invention relates to a surface acoustic wave device in which an IDT electrode and a dielectric film are disposed on a piezoelectric substrate.
2. Description of the Related Art
Surface acoustic wave devices have been widely used as a resonator or a band-pass filter of communication apparatuses heretofore.
For example, the following Japanese Unexamined Patent Application Publication No. 2008-522514 discloses a surface acoustic wave device in which an IDT electrode and a dielectric film containing SiO2 are laminated on a piezoelectric substrate. Japanese Unexamined Patent Application Publication No. 2008-522514 discloses that the temperature characteristics can be improved by forming the SiO2 film so as to cover the IDT electrode to thereby reduce the absolute value of the frequency temperature coefficient TCF. Japanese Unexamined Patent Application Publication No. 2008-522514 also discloses that the IDT electrode has a structure in which various metal films are laminated, and therefore the reflection intensity of surface waves can be adjusted by controlling the film thickness of each metal film in a specific range. The film thickness of the dielectric film containing SiO2 is about 20% to about 40% of the wavelength λ of surface acoustic waves.
However, in the surface acoustic wave device of Japanese Unexamined Patent Application Publication No. 2008-522514, since the insertion loss is not low, it is required to further reduce the insertion loss. Moreover, the thickness of the dielectric film is relatively as thick as about 20% to about 40% of the wavelength. Therefore, the electromechanical coupling coefficient is not so high. When used as a band-pass filter, it has been difficult to broaden the bandwidth.